This invention relates to a rotor for a superconducting rotating electric machine.
A typical rotor of the type with which the present invention is concerned is illustrated in FIG. 1 in cross-sectional form. As shown in the figure, cylindrical torque tubes 1 are rigidly secured to opposite ends of a hollow, cylindrical coil-carrying shaft 2 on which superconducting field coils 3 are mounted. The right torque tube 1 in the figure is rigidly secured to a first end shaft 8, and the left torque tube 1 in the figure is rigidly secured to a second end shaft 9 which is drivingly connected to a prime mover or to a load, depending upon whether the rotor is to be used as part of a generator or a motor. The first end shaft 8 and the second end shaft 9 are both rotatably supported by bearings 10. The second end shaft 9 has a number of slip rings 11 mounted thereon via which field current is supplied to the field coils 3.
The coil-carrying shaft 2 is surrounded by a cylindrical warm damper shield 4 whose opposite ends are secured to the first end shaft 8 and the second end shaft 9. A cylindrical cold damper shield 5 is disposed between the warm damper shield 4 and the coil-carrying shaft 2 with a longitudinally-extending space therebetween. The warm damper shield 4 and the cold damper shield 5 shield the field coils 3 from high-frequency magnetic fields and decrease rotor oscillations due to disturbances in the electrical power system to which the rotor is connected. In addition, the warm damper shield 4 forms a vacuum seal between the inside and outside of the rotor, and the cold damper shield 5 acts as a radiation shield for the inside portions of the coil-carrying shaft 2 in which liquid helium is contained.
A helium outer tube 6 surrounds the coil-carrying shaft 2 with a space left between its outer surface and the inner surface of the cold damper shield 5. The ends of the coil-carrying shaft 2 are sealed by end plates 7, and the central cavity 15 of the coil-carrying shaft 2 is filled with liquid helium.
Heat exchangers 12 are either disposed on or formed as part of the torque tubes 1. At either end of the coil-carrying shaft 2, lateral radiation shields 13 are provided which protect the field coils 3 from lateral radiation. A vacuum is maintained in the spaces 14 between the warm damper shield 4 and the cold damper shield 5, between the cold damper shield 5 and the helium outer tube 6, and between the outer ends of the coil-carrying shaft 2 and the second end shaft 9 and the first end shaft 8.
In a typical rotor for a superconducting rotating electric machine of the illustrated type, liquid helium is introduced into the central cavity 15 of the coil-carrying shaft 2 through unillustrated pipes which pass through the second end shaft 9. The liquid helium cools the superconducting field coils 3 to cryogenic temperatures at which their electrical resistance is zero. As a result, there are no excitation losses, and a powerful magnetic field can be generated by the field coils 3 to generate alternating current electric power in an unillustrated stator. The direction of flow of the liquid helium through the rotor is indicated by the arrows.
FIG. 2 illustrates in greater detail the end portion of the coil-carrying shaft 2 of FIG. 1, showing the structure of a rotor disclosed in Japanese Laid-Open Patent Application No. 57-13961. The field coils 3 are housed in a longitudinally-extending recess 2a formed in the coil-carrying shaft 2 and are secured against centrifugal forces by a retaining ring 16 which is shrink fit over the coil-carrying shaft 2. A longitudinally-extending space 19 is left between the outer surface of the retaining ring 16 and the inner surface of the helium outer tube 6. This space 19 is filled with liquid helium.
The field coils 3 are separated from one another, from the sides of the recess 2a in which they are housed, and from the inner peripheral surface of the retaining ring 16 by electrically-insulating spacers 17. In addition to providing electrical insulation, the spacers 17 prevent the lateral movement of the coils 3.
A radially-extending hole 18 is formed between the central cavity 15 of the coil-carrying shaft 2 and the radially-inner surface of the recess 2a in which the field coils 3 are housed. This hole 18 is in fluid communication with unillustrated grooves which are formed in the spacers 17 so that liquid helium can flow from the central cavity 15 into the grooves in the spacers 17 through the hole 18, thereby cooling the coils 3.
The conventional structure shown in FIG. 2 has the drawback that it is possible for heat which is conducted along the torque tube 1 to flow into the coil-carrying shaft 2 and to reach the coils 3. The flow of this heat is illustrated by the arrows in the figure. Although some of this heat is absorbed by the liquid helium in the hole 18 and in the unillustrated grooves in the spacers 17, the amount of heat which reaches the coils 3 is still significant enough that it can lead to a breakdown in the superconductivity of the field coils 3. If such a breakdown in superconductivity occurs, it is necessary to stop the rotation of the rotor. When the rotor is employed in a generator, a loss of superconductivity is especially serious and can require the shutting down of an entire generating unit.
As a means for decreasing the amount of heat which reaches the field coils 3 by conduction along the ends of the coil-carrying shaft 2, it has been proposed to increase the number of radially-extending holes 18 between the central cavity 15 and the recess 2a of the coil-carrying shaft 2. However, the machining of a larger number of holes 18 requires a great deal of effort, and because of the location of the holes 18 with respect to the coils 3, their cooling efficiency is not necessarily good. Accordingly, increasing the number of holes 18 is not a satisfactory solution to the problem.